Aircraft ground-speed indicator



April 3, 192s.

F. L. HUNT ET AL AIRCRAFT GROUND SPEED INDICATOR Filed not. 19, 192s lm/vento@ 77. f 7M UNITED STATEsPA-TENT OFFICE..

FRANKLIN L. HUNT ANI) ROY C. SYLVANDER, OF WASHINGTON, DISTRICT OF COLUMBIA.'

i AIRCRAFT GROUND-SPEED INDICATOR.

Application led October 19, 1923. Serial No. 669,618.

This invention relates to ground speed measuring devices and more particularly. to such as employ a rotating prism for viewlng purposes of the ground traversed.

xIrl'eretofore in measuring ground speeds, when at a given altitude, a rotating prism has been employed to produce a drift component in the optical iield of a telescope or the like, in order to compound with the natural drift acrossthe optical field produced by the horizontal motion of the instrument itself over 'the surface of the ground. Such a method of measuring ground vspeeds is ,not

very accurate principally .due to the fact that the ground velocity to be derived depends on the evolution of the trigonometric tangent produced by the compounding of the respective velocity vectors.

In the present instance, however, a null method is employed which does away with the above inaccuracies of evolution. Thus instead of producing a drift, the principal object of the invention is to nullify all objects of drift and particularly the eifect of the natural drift due to the lateral translation of the deviee For a given height therefore, it will at once follow that the ground speed is determinable by the speed of revolution necessary to be impressed upon the rotating prism.

In order to correct for varying heights above the ground, whichnaturally aifect the speed with which the ground View appears to pass across the field of vision, the tachometer device measuring the revolutions .of the rotating prism canbe automatically adjusted. Thus, for purposes of this invention a gear multiplication or diminution can be employed. In the present instance this is automatically provided for by adjustably arranging a friction roller' between two displaced engaging discs. y

However, any other type of variable speed control can be employed offering a means forfadjustably arranging the mechanical advantage.

In order more completely to disclose our f' present invention the following drawings are attached, in which;

Figure 1 represents a plan v1ew of the instrument; l

Figure 2 'corresponds to a part sectional view of Figure 'l' snbstantlally` along the line 2-2,

' the telescope 3 Figure 3 similarly to Figure 2 corresponds to a part sectional view substantially along the line 3-3 of Figure 1. g

1Within a casing 1 a prism 2 is revolvably mounted within a telescope tube 3 through the latter of which it is aimed to view Jthe ground or territory over which passage is being taken. A driving shaft 4 is coupled by means of. a reduction disc or the like 6, eccentrically displaced from a second friction disc 7 or the like, between which is arranged afriction roller or the like 8, mounted adjustably on a screw member '9 which is hand controlled by the knob 10 through suitable gearing 11; vAs a consequence of the latter arrangement it is possi ble to secure any desired amount of rotative speed of the prism 2 by merely turning the knob 10. Thus, in operating the instrument is directed perpendicularly toward the ground or territory passed over, and such a speed is impressed upon the prism that when viewed from the eye-piece of the telescope 3 the ground will appear to remain substantially stationary.

In order to measure the required speed of rotation of the prism, to give the substantially stationary effect a further roller 12 is `provided to engage with` the disc member 7 so as to give rotativity to a further friction disc 13 or the like, by virtue of a mechanical train 14. It is the disc 13, which virtually impresses upon the tachometer the mechanical train, schematically represented by 15 in the figures, the required xrotativity to give a reading on the tachometer dial (see Figure 1) the noted ground speed in miles per hour for the assumed standard elevation of the instrument from the ground. n

In viewof the fact that at'fhigher distances from the earthssurface theground appears to be traversing the optical field at less and less speed, it is advisable to automatically correct for this effect, by introducing a variable speed drive between the actuated disc 13 synchronously operating in fixed ratio with respect to the prism disc 7 on the one hand, and on the other hand with the train symbolized by` 15 actuating the tachometer. For this purpose, therefore, engaging with the disc 13 is provided a friction roller or the like 17 adjustably mounted on a spindle 18, whose fixed pinlon 19 emgages withl a tachometer train 19. In order to produce gearing 5 to a friction this adjustability a movable rack 20 is mounted in the casing of the instrument so that through a fixedly mounted pinion drive 21, '22 engagement is made with a movable n sleeve 23 having a rack 24 on one-side, ca-

` pable of moving the friction roller 17 with respectto a spline formed integrally on the spindle 18 so as'to alter the position of the roller with :respect to thev engaged surface of the disc 13. A portion ofthe rack 20 is adjustably controlled from the outside of strumentmtoward, the ground, with the axes` the instrument by means'of a knurled button I l of rotation of the prism, transverse tothe direction oftravel, the speed adjusting knob 10 isfturned whether to right or left to give suitable prism speed to cause theoptical field to -appear substantially stationary. With proper adjustments thus arrived at the tachometer will registeren its dial 16 the proper ground speeds of the operator.v

Having described the nature of our inyvention .what we claim is 1. In combination with a viewing piece, a prism, means'for rotating said prism in the field of said ,viewing piece to produce a,

substantially stationary ield of view, and means to determine the speed of rotation of said prism, and means between the said prism and thesaid speed determining means inesatta shiftable in accordance with altitude to alter the indicated speed of the said prism.

2. In combination with a viewing piece, a prism, means for rotating said prism at variable speeds froml a constant speed source.

in the field of said viewing piece, to produce a substantially-stationary field of view,

and means to determine the speed of rotation required to produce such field of view', and means between the 'said prism and the said speed determining means shiftable. in accordance. with altitude 4to alter the indicated speeds ofv theasaid prism. M

' 3. In combination with a viewing piece,4

a prism, means for rotating said prism from a constant speed source, in the ieldof saidviewing piece to producea substantially stationary iield of view, tachometer. means to determinevthe required speed of rotation, further means for adjustably altering the speed effect on said tachometer means to correct for altitude of saidviewing piece and means between the said prism. and the said 'speed determining means shiftable in accordance with altitude to alter the indicated 'speed of said prism.

4. In combination with a rotatable viewing prism and a rotating means therefor,-

means to vary the speed` of rotation of said prism to any required degree from said rotating means, a' tachonieter means drivenby said rotatingmeans at a speed having a given ratio vto the speed of rotation of said prism' and means longitudinally shiftable in accordance with altitude to variabl atleet said tachometer means by altering t e ratio pf speeds of rotation of said tachometer means and said prism. FRANKLIN L HUN'D BUY C. SYMVAlNlDERo 

